Zinc oxide is a well known component of coatings for automotive and architectural glazings. Such coatings are useful for providing enhanced performance characteristics, e.g., anti-reflectivity, electrical resistivity, infrared energy rejection, etc. A zinc oxide layer may conveniently be deposited onto a transparent substrate, either singularly or in combination with other metal and dielectric layers, by conventional methods such as, for example, spray pyrolysis, sputtering, vacuum evaporation, or chemical vapor deposition.
A zinc oxide layer which is particularly useful for preparing high performance automotive and architectural glazings is formed by the spray pyrolysis process, in which a reactant mixture containing a soluble zinc salt is sprayed onto a hot glass ribbon being produced by the float glass process. The zinc-containing compound pyrolyzes at the surface of the glass and, in the presence of oxygen, produces a layer of zinc oxide thereon. Several factors are important in determining the thickness, uniformity, and chemical durability of the resultant zinc oxide layer. The temperature of the substrate must be high enough to effect the pyrolysis reaction, generally at least about 900.degree. F. for the deposition of zinc oxide. The reactant mixture may be diluted with organic solvents or water for ease of control. However, the degree of dilution is limited by the need to attain a useful film deposition rate, and also by the chemistry of the pyrolysis reaction itself since the solubility limit must not be approached under the solution in-flight conditions. The spray pattern and velocity distribution of the droplets, as well as the turbulence and lateral vapor movement at the surface of the substrate, likewise affect the quality of the deposited layer. Finally, the zinc oxide layer, which otherwise would readily dissolve in acid and alkaline solutions, can be made more durable by codepositing other metal oxides.
A particularly useful and well-known zinc-containing pyrolytic precursor is zinc acetate. See, for example, Major et al., "Highly Transparent and Conducting Indium-Doped Zinc Oxide Films by Spray Pyrolysis," Thin Solid Films, 108 (1983) 333-340. Spray pyrolysis utilizing zinc acetate results in a high quality film having a uniform appearance, high optical transmittance, and good electrical resistivity. Zinc oxide layers produced utilizing zinc acetate comprise columnar crystallites with average diameters of about one Angstrom and heights equal to the film thickness.
U.S. Pat. No. 3,850,665 to Plumat et al. discloses a process for forming a metal oxide coating on a substrate, wherein an acetyl acetonate coprecipitate of two or more metals such as, for example, zinc and chromium is disolved in a solvent and sprayed onto a hot glass ribbon. The resultant coating is a metal oxide containing zinc chromite (ZnCr.sub.2 O.sub.4). The patent does not teach the addition of an acetyl acetonate to a zinc acetate-containing solution for spray pyrolysis, for increasing the chemical durability of a zinc oxide layer.
U.S. Pat. No. 4,292,347 to Donley discloses a pyrolytic coating reactant for producing metal oxide films, comprising a metal carboxylate such as, for example, zinc 2-ethyl hexonate and a diketonate such as, for example, chromium acetyl acetonate. The patent states that the combination of these pyrolytic coating reactants improves the durability of the produced coating.
Finally, U.S. Pat. No. 4,239,816 to Breininger et al. discloses that a mixture of iron, chromium, and cobaltous acetyl acetonates, when pyrolytically applied to a glass surface, results in a more durable metal oxide than if any of the acetyl acetonates were to be pyrolytically deposited alone. The patent does not disclose the use of acetyl acetonates in conbination with zinc acetate, to produce a layer of zinc oxide.
In summary, the prior art does not suggest that spray solution adjuvants may be useful to increase the chemical durability for zinc oxide layers deposited by a spray pyrolysis process utilizing zinc acetate as the zinc-containing pyrolysis reactant mixture precursor.
It would be desirable to increase the chemical durability of a zinc oxide layer deposited onto a hot glass ribbon utilizing a spray pyrolysis process, wherein zinc acetate is the zinc-containing pyrolytic precursor. Zinc oxide layers produced utilizing zinc acetate as the pyrolytic precursor have superior optical and physical properties, but suffer poor chemical durability when exposed to acid and alkaline solutions.